1. Field of the Invention
The invention relates to an arrangement for retroreflection of an optical ray using triple prisms.
2. Description of the Related Art
In geodetic surveying, in construction surveying and in industrial measurement technology use is made of self-radiating or reflecting aiming markers such as spherical reflectors, reflecting foils or triple prisms, in order to mark points in space. The aiming markers are sighted with theodolites or with a tachymeter, that is to say a theodolite with a range finder. By measuring direction and distance to the aiming markers, their three-dimensional co-ordinates are determined with regard to a prescribed co-ordinate system.
In addition to determining and recording co-ordinates, it is also possible conversely for co-ordinates, which are recorded in a plan or can be taken from a map, to be laid out on the spot. For example, in roadbuilding the line is laid out by means of theodolite or a tachymeter and a plumb rod provided with an aiming marker. In this case, at least two people are required as operators who communicate by hand sign or radio. On the one hand, the co-ordinates prescribed according to the plan are set on the surveying instrument while, on the other hand, the plumb rod with the aiming marker is moved until the aiming marker has reached the required co-ordinates. In this case, the operators always have to pay heed to the alignment of the aiming marker relative to the surveying instrument.
In the case of surveying operations in which the instrument setups change, the same bench marks are frequently used for surveying. The aiming markers set up for the bench marks are sighted in this case from different directions. The aiming markers must, in turn, be aligned with the respective surveying instrument by rotation.
Furthermore, in the case of specific surveying operations in building or surveying, an aiming marker can serve simultaneously as a reference aiming marker for a plurality of surveying instruments. In this case, as well, the aiming marker must successively be aligned with each individual surveying instrument.
A further difficulty arises for the carrier of the plumb rod with the arrival of automated surveying instruments. With modern surveying instruments, the visual observation of the aiming marker via the theodolite telescope is additionally supported or even replaced by an electronic recording of the aiming marker. The recordings are performed by video cameras or with the aid of CCD-Arrays which are integrated in the imaging optical system of the theodolite. Such systems are described in the journal "OPTICUS", No.1/94, Pages 8 to 9, of the company LEICA AG, "Theodolite--Sensoren fur die industrielle Vermessung" ("Theodolites--Sensors for industrial surveying") or in DE 3424806 C2. It is possible with the aid of image processing automatically to detect the aiming markers and determine their co-ordinates. As a result, a motorized theodolite can be controlled in such a way that it can automatically track a moving aiming marker. This means that the carrier of the plumb rod can certainly move freely in the field without the theodolite losing the aiming marker from the field of view. However, the aiming marker must simultaneously always be aligned with the theodolite, and this deflects the attention of the carrier of the plumb rod from the field and the measuring points. Consequently, for this application of the automated tracking of aiming markers the need also arises to have an aiming marker with a relatively large, retroreflecting angular range.
The spherical reflectors, chromium-plated steel balls, frequently used in industrial measurement technology would be ideal. They can be illuminated from all directions and deliver the same aiming marker image in each case. However, only very short ranges can be measured with them, since the intensity reflected by them decreases with the fourth power of the distance. Consequently, in geodetic surveying or in construction surveying use is mostly made of reflecting foils or triple prisms, which have a high reflection factor. However, with them the retroreflected intensity is a function of angle, and the useful angular range is limited to approximately .+-.45.degree..
U.S. Pat. No. 5,301,435 has disclosed a prism arrangement in which a plurality of retroreflecting prisms, which have a customary round light entry surface, are arranged on a cylindrical surface. The individual prisms are spatially separated. Three prisms each are situated in two planes lying one upon another. The prisms of one plane are offset by 120.degree. relative to one another and by 60.degree. with respect to those of the neighboring plane. Consequently, each prism covers an angular range, and the entire prism arrangement reflects light from all horizontal directions over 360.degree.. Distances are measured with this prism arrangement. However, in this case only distances above a specific minimum distance of approximately 1000 m can be measured.